Vegetable Disease Update – 9/20/13

Carrots – Leaf blights – Powdery mildew, Alternaria and Cercospora. Alternaria and Cercospora are two soil-borne fungal pathogens that may cause early defoliation in carrots reducing yields and making harvest difficult. Both pathogens produce distinct symptoms on carrots. Powdery mildew causes characteristic white, powdery lesions on foliage. Symptoms of Alternaria include irregular, dark brown to black spots which typically show up on older leaves first. Cercospora leaf spots are round, grayish-brown and are more prevalent on younger foliage. Both leaf blights typically start at the margins of leaflets and as more spots develop leaflets begin to wither and die. Symptoms similar to leaf infections can develop on stems and petioles. Control of both diseases begins with regular scouting and preventative fungicide applications on susceptible varieties. Apply Quadris (azoxystrobin, 11) at 9.0 to 15.5 fl. oz 2.08F/A, or Cabrio 20EG (pyraclostrobin, 11) at 8.0 to 12.0 oz/A, or Pristine (pyraclostrobin + boscalid, 11 +7) at 8.0 to 10.5 oz 38WG/A tank-mixed or alternated with chlorothalonil (M5) at 1.5 to 2 pt/A. For Alternaria only: Apply a tank mix of 1.5 to 2.0 pt/A chlorothalonil plus Endura (boscalid, 70 at 4.5 oz 70W/A, or Switch (cyprodinil + fludioxonil, 9 +12) at 11.0 to 14.0 oz 62.5WG/A, or iprodione at 1.0 to 2.0 pt 50WP/A. For more information on tolerant varieties and control please the 2013 New Jersey Commercial Vegetable Production Recommendations.

Cole crops – Downy mildew and Alternaria – Symptoms of downy mildew include purple to yellowish-brown spots on upper leaf surfaces. A grayish-white spore mass will develop and cover the underside of leaves under ideal temperatures (night temperatures of 46 to 61oF and day temperatures below 75oF). Downy mildew can kill young plants. Heavily infected leaves may drop providing entry points for bacterial infections (i.e., black rot and soft rot). Symptoms of Alternaria on infected leaves include small, expanding circular lesions with concentric rings that may have a ‘shot-hole’ appearance as lesions age. Heavily infected seedlings may result in damping-off. Control of Downy mildew and Alternaria begin with preventative fungicide applications. Use one of the following at the first sign of disease and continue every 7 to 10 days (Please refer to the pesticide table on page F24 of the 2013 NJ Commercial Vegetable Production Recommendations to determine which fungicide is labeled for each specific crop.): Quadris (azoxystrobin, 11) at 6.0 to 15.5 fl oz 2.08SC/A, or chlorothalonil (M5) at 1.5 pt 6F/A or OLF, or Cabrio (pyraclostrobin, 11) at 12.0 to 16.0 oz 20EG/A, or Endura (boscalid, 7) at 6.0 to 9.0 oz 70WG/A, or Ridomil Gold Bravo (mefenoxam + chlorothalonil, 4 + M5) at 1.5 lb 76.5 WP/A, or Switch (cyprodinil fludioxonil , 9 +12) at 11.0 to 14.0 oz 62.5WG/A (Alternaria only), or Fontelis (penthiopyrad, 7) at 14.0 to 30.0 fl oz 1.67SC/A (Alternaria only). For downy mildew only, apply Actigard (acibenzolar-S-methyl, P) at 1.0 oz 50WG/A (begin applications 7-10 days after thinning and re-apply every 7 days for a total of 4 applications per season), or Aliette (fosletyl Al, 33) at 3.0 to 5.0 lb 80WDG/A (on 14-day schedule).

Lettuce –Bottom rot/Drop – For Bottom Rot, Endura 70WG (boscalid, 7) at 8.0 to 11.0 oz/A, or iprodione (FRAC code 2) at 1.5 to 2.0 lb/A should be applied one week after transplanting or thinning and 10 and 20 days later. For Drop, the biological Contans 5.3WG at 2.0 to 4.0 lbs/A pre-plant can be incorporated at a depth of 1 to 2 inches. Apply Cannonball (fludioxonil, 12) at 7.0 oz 50WP/A, or iprodione (FRAC code 2) at 1.5 to 2.0 lb/A, or Endura (boscalid, 7) at 8.0 to 11.0 oz 70W/A, or Quadris (azoxystrobin, 11) at 0.40 to 0.80 fl oz. 2.08SC/A beginning one week after transplanting or thinning and again at 10 and 20 days later. For more information on control of Bottom Rot and Drop and other important diseases of lettuce please see the 2013 New Jersey Commercial Vegetable Production Recommendations Guide.

Pumpkin – Sunscald injury – Sunscald injury occurs when pumpkin fruit are suddenly exposed to heavy sunlight during the latter stages of fruit ripening during the fall. Sunscald injury often occurs when pumpkin plants become prematurely defoliated in the early fall by powdery mildew or downy mildew or when vines collapse due to Phytophthora blight or bacterial wilt. Symptoms of sunscald injury include the collapsing of rind tissue on the side of the fruit which is in direct contact with the afternoon sun. Sunscald injury often develops as a pinkish-red color on exposed fruit which becomes flat in appearance. Over time fruit tissue may become tan to brown and secondary pathogens often invade the sunscald injured areas of the fruit. To help reduce the potential for sunscald injury, maintain foliage for as long as necessary, especially if fruit are going to be left in the field for long periods.

Pumpkin – White mold or Sclerotinia rot – White mold may cause problems when pumpkins are planted in the same field each year and in fields where other susceptible crops such as bean have been grown. Development of white mold is favored by prolonged, cold wet weather. Symptoms often begin to show up as a soft, mushy area around the stem as the fruit reaches maturity. Infected fruit often collapse inward near the stem. Large, black fruiting bodies (sclerotia) may be produced around infected areas. Sclerotia serve as overwintering and long-term survival structures. A long crop rotation is necessary to help control white mold. Infected fruit should be removed from the field immediately. Early maturing fruit left in the field for a prolong time period are susceptible to white mold.